1. Field of the Invention
This invention relates to magnetic recording apparatus for recording a video signal and, more particularly, to such apparatus in which the video signal is recorded in successive tracks with proper framing, even at an edit point.
2. Description of the Prior Art
In one type of video tape recorder, such as an 8 mm format VTR, video signals are recorded in successive, contiguous tracks by two (or more) rotary transducers, or heads, which scan an arcuate extent of magnetic tape greater than 180.degree.. FIG. 1 illustrates the relationship between the rotary heads and the magnetic tape in one embodiment; and FIG. 2 is a schematic representation of the record tracks recorded by this apparatus. From FIG. 1, it is seen that heads 1A and 1B are angularly separated from each other by 180.degree. and scan alternate tracks across magnetic tape 2 which has a wrap angle about a guide drum that is greater than 180.degree.. The tape advances over the surface of the guide drum in a diagonal direction, resulting in the recording of alternate contiguous tracks 3A and 3B, shown in FIG. 2.
As is conventional, the magnetic recording gap of head 1A is disposed at an azimuth angle different from that of head 1B. By using different azimuth angles, the well-known phenomenon of azimuth loss is relied upon to minimize undesired cross talk that may be picked up during reproduction. If it is assumed that head 1A records tracks 3A and head 1B records tracks 3B, then during a playback operation when head 1A scans track 3A, minimal cross talk is picked up by this head from adjacent tracks 3B. Likewise, because of azimuth loss, when head 1B scans a track 3B, minimal cross talk signals are picked up by this head from adjacent tracks 3A.
In a typical embodiment, each head records a video signal field interval in one track; and if two heads are used, as illustrated in FIG. 1, then one complete rotation of the heads results in the recording of a video frame. The heads thus are rotated at a rate equal to the video signal frame repetition rate which, of course, is in synchronism with the usual vertical synchronizing signal. Thus, head 1A records and reproduces alternate tracks 3A; and head 1B records and reproduces the remaining tracks 3B.
When recording video signals on magnetic tape 2, the information represented by those signals may be derived from separate sources, such as separate video cameras, separate video playback devices, or the like. Typically, the change over from one source to another is effected at the end of a field interval and not in the middle of that interval. Consequently, one may think of a so-called joint or edit point recorded on tape 2, wherein the track which precedes this joint or edit point contains video signals derived from one source and the track which immediately follows this joint or edit point contains video signals derived from a different source. Preferably, the joint or edit point always is provided such that track 3A contains video signals derived from the aforementioned first source and track 3B contains video signals derived from the second source. With this limitation, the continuity (or periodicity) of tracks 3A and 3B is maintained, even across a joint or edit point.
Although not shown in FIGS. 1 and 2, when successive tracks are recorded in accordance with the 8 mm VTR format, a pilot signal is recorded in each track together with the video signals. As is known to those of ordinary skill in the art, the frequency of the pilot signal is such that it may be superimposed onto the video signal without disturbing the video picture or sound which subsequently is reproduced. Typically, the frequency of the pilot signal is well below the frequency spectrum of the recorded video signals.
This pilot signal is used during a playback mode for controlling the tracking of the transducers such that transducer 1A is aligned with track 3A and transducer 1B is aligned with track 3B. Accordingly, the frequency of the pilot signal is changed from track to track; and this change occurs cyclically so that a respective pilot frequency is recorded in each of four successive tracks. Hence, the period of recurrence of the pilot signal frequency is equal to four field periods. If the pilot frequency recorded in a track is represented as f.sub.i, then the relationship between the pilot frequency, the particular recording head which is used to record that pilot frequency superimposed onto the video signal and the field period of this video signal is shown in the following table:
TABLE 1 ______________________________________ Field Period Pilot Frequency f.sub.i Recording Head ______________________________________ 1 f.sub.1 = f.sub.H .times. 378/58 = 1A 103 kHz 2 f.sub.2 = f.sub.H .times. 378/50 = 1B 119 kHz 3 f.sub.3 = f.sub.H .times. 378/36 = 1A 165 kHz 4 f.sub.4 = f.sub.H .times. 378/40 = 1B 149 kHz ______________________________________
In the foregoing table, f.sub.H represents the horizontal frequency, and in the NTSC system f.sub.H =15.734 kHz.
From Table 1, it is appreciated that in the conventional 8 mm format, head 1A records a pilot frequency f.sub.1 or f.sub.3 and head 1B records a pilot frequency f.sub.2 or f.sub.4.
The phase relationship between the vertical synchronizing pulse and the horizontal synchronizing pulse in the NTSC system is such that a given state (or relation) recurs every two fields. For example, in the first field of a frame (usually identified simply as field one), the vertical blanking interval is spaced from the immediately preceding horizontal synchronizing pulse by one full line interval, and in the second field of that frame the vertical blanking interval is spaced from the last preceding horizontal synchronizing pulse by one-half of a line interval. After two field intervals, the vertical blanking interval once again is spaced from the last preceding horizontal synchronizing pulse by a full line interval.
Although the continuity or periodicity between tracks 3A and 3B is maintained as aforesaid, it is possible that the video signal which is recorded in the track immediately preceding the joint or edit point is an odd-numbered field, such as the first field of a frame, and the video signal recorded in the track immediately following the joint or edit point also is an odd-numbered field. Conversely, the video signal recorded in the tracks on either side of the joint or edit point may be an even-numbered field, such as the second field. Consequently, the fields recorded in successive tracks may appear as "odd-even-odd-odd-even . . . " or "odd-even-odd-even-even-odd-even . . . ". Because of the different phase relationships between the vertical and horizontal synchronizing pulses in odd and even fields, if successive odd-numbered fields (or even-numbered fields) are recorded and subsequently played back, the reproduction of such successive fields will produce a disturbance in the continuity of the vertical and/or horizontal synchronizing pulses. That is, when successive odd-numbered fields or successive even-numbered fields are reproduced, a disturbance in the framing of the video picture produced therefrom will occur. Unless steps are taken to prevent an odd-numbered field (or even-numbered field) from being recorded on both sides of a joint or edit point, the reproduced video picture will contain a disturbance at that joint or edit point.
When video signals in the PAL system are recorded, the resultant disturbance produced in the continuity of the vertical or horizontal synchronizing pulse when the joint or edit point is reproduced adversely affects the usual AFC/APC circuits of a typical monitor/receiver. Thus, not only is a framing disturbance produced in the reproduced PAL video picture, but the color quality of that picture tends to be degraded.